This invention relates to high strength, low alloy (HSLA) steels having low carbon content and having good mechanical properties, i.e. by tensile and toughness measurements, particularly in respect to ductility and formability, for instance in exhibiting the ductility required for bending. The invention is particularly concerned with steel products which can be of the nature of sheet or the like, achieved by hot deformation; the contemplated products are thus made by hot rolling to desired thicknesses and shaped, notably strip, which can be used as so produced or as subsequently further reduced by cold rolling to sheet form, i.e. strip or the like, of thinner gauge.
The present improvements are notably designed to afford a steel of very low carbon content, with excellant properties in both longitudinal and transverse directions in reference to hot rolling. The steels are in the broad category defined by yield strengths of 60 ksi (60,000 pounds per square inch) and above, and in a particular sense the invention is concerned with steel products having yield strengths in the range of 70 ksi and higher, including a category of 70-80 ksi, and a category of greater strength, even up to 95 ksi. Indeed, a special aspect of the present invention resides in the provision of new and improved high strength low alloy steels, particularly as hot rolled strip, having yield strength of 80 ksi or better, preferably up to 90 ksi.
The demand for steel products of the nature described above rests to a considerable extent on increasing need for high strength in steel strip, sheet and the like, with a minimum of weight and, understandably, at as little cost as possible. For example, steels of this nature have many uses in vehicle constructions, particularly in the automotive area where for fuel economy it is desirable to reduce the weight of the structure, yet without impairing strength.
A basic purpose of HSLA steels of this class is therefore to achieve high strength properties, with a minimum of alloying elements and a minimum of processing expense. At the same time, however, it has been difficult to obtain satisfactory products in a variety of respects without employing a number of special elements for different purposes. Thus, high tensile properties and toughness can be obtained with additions of certain elements to steels of moderate to high manganese content and moderately low carbon, but avoidance of directionality in some of these properties, such as toughness and bendability, has usually required further additions, exemplified by rare earth elements, as well (in some cases) as special desulfurization.
Not only have the further additions, just mentioned, contributed to the cost of the described steels, but there appears to have been room for improvement in cost reduction even as to the quantity of other elements included. With previous efforts toward economy, useful HSLA steels have been difficult to attain at the higher strength levels and with realization of practical characteristics, e.g. such as nondirectionality, good toughness, and good weldability of the ultimate products. Attention is also needed to the problem of convenience in processing these steels, in hot rolling operations. In many cases of previous lower-cost HSLA compositions, very careful control has been required for finishing temperatures, coiling temperatures, and the like, within narrow ranges. Likewise, it has appeared that hot rollability is not always as good as might be desired, particularly in that some strength-promoting or other elements of the composition are believed to stiffen the hot band during hot rolling; if possible, there has been a need to reduce the hardness factor, now found to result, for example, from higher additions of columbium.